This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This Phase III study is supported by a National Institute on Aging grant. The purpose of this study is to determine whether chronic DHA supplementation slows the progression of cognitive and functional decline over 18 months in mild to moderate AD. Endpoints of this study are scores on the Alzheimer's Disease Assessment Scale (ADAS-Cog) and the Clinical Dementia Rating Scale Sum of Boxes (CDR-SOB). Several biomarkers are included as secondary outcome measures. Plasma and urine biomarkers for anti-amyloid, antioxidant, and neuroprotectant effects were included because of evidence from animal studies that DHA acts via these mechanisms. A subgroup of study subjects will be asked to participate in a magnetic resonance imaging (MRI) sub-study, in which MRI scans will be used to make volumetric measurements of the brain. Another subgroup will be asked to participate in a lumbar puncture (LP) sub-study, to assay CSF A? tau, P-181 tau, and lipid peroxidation products (isoprostanes).